Patent GB-763615-A (GB)
Title: Uhf-vhf tuners
IPC: H03J5/02
Filed: 1953-11-18
Abstract: 763,615. Radio receiving apparatus; automatic gain control; valve oscillators. STANDARD COIL PRODUCTS, Inc. Nov. 18, 1953 [Feb. 4, 1953], No. 32000/53. Classes 40(5) and 40(6) Tuning apparatus for use in a multichannel television receiver comprises a nine-position turret carrying a plurality of coils and adapted to select any one of a plurality of groups or bands of contiguous channels, and a fine tuning mechanism comprising a plurality of ganged variable condensers for selecting by a continuous tuning operation, any one channel from among the group or band to which the turret is set. The first four turret positions correspond to channels in the VHF range covering 54 to 216 Mc/s while the remaining five positions correspond to channels in the VHF range covering 470 to 890 Mc/s. Over the four VHF bands the turret connections are so arranged that the radio frequency portion of the tuner circuit comprises a cascode amplifier 15, Fig. 1, preceded by one R.F tuned circuit 14 and followed by three R.F. tuned circuits 16. The output from the circuits 16 is applied through the mixer 17 to a single I.F. tuned circuit 19 and thence to the remaining I.F. and video and sound stages of the receiver. When however the turret is set to receive over any one of the five U.H.F. bands, the turret connections are so arranged that the cascode amplifier appears in the I.F. stages of the receiver. The R.F. section of the tuner comprises three R.F. tuned circuits 255, Fig. 2, followed by the crystal mixer 2, followed by the crystal mixer 256 (employing the same components as the mixer 17, Fig. 1), which is followed by two I.F. tuned circuits 258, the cascode amplifier now tuned to I.F. and bearing the reference numeral 260 and a final I.F. amplifier 261 leading to the remaining circuits of the receiver. The circuit arrangement for the V.H.F. bands is shown in Fig. 3, the circuit being tuned to the desired band by the components on the panel 40 forming part of the turret, the tuner output being taken by the co-axial cable 202. The four tuned R.F. circuits comprise pairs of condensers 30, 31, 93, 94, 118, 120 and 131, 132 of which the first three pairs are variable. The oscillator 18 comprises variable condensers 183, 184. The circuit arrangement for U.H.F. wherein the panels 40 are replaced by differently arranged panels 70, is shown in Fig. 4 the three R.F. tuned circuits employing condensers 93, 94, 118, 120, and 131, 132, and the oscillator 257 being tuned as before by the condensers 183, 184. In this case the mixer output leads to the cascode amplifier 260, the output of which leads through the tuned circuit 261 to the co-axial cable 202. The tuner is shown in side view in Fig. 9 and in transverse section in Fig. 10. The nine positions of the turret are located by an indented disc 425 and spring-urged roller 427. The tuning condenser arrangement comprises a plurality of shaped dielectric elements 452 (see also Fig. 13, not shown) mounted on a conductive cylinder 454 mounted on a shaft 455. Rotation of the cylinder is effected by means of a sleeve 482 mounted on the shaft 415 of the turret through the agency of a pair of discs 480, 481 which frictionally engage a disc 475 mounted on the shaft 455. The dielectric elements 452 co-operate with insulated stator plates 450, 451 and also with earthed elements 470 which carry contacts 471 bearing against the conductive cylinder 454. Limitation of movement of variable condensers. Since each U.H.F. band comprises the same number, fourteen, of channels,. the total angular extent of the tuning range of the condenser arrangement will be less on the higher ranges than on the lower. To allow for this, a disc 456 carrying a pin 457 is attached to the shaft 455, (see also Fig. 12) and the pin engages with a bentover portion at the upper end of a plate 485 which is positioned in accordance with the turret setting by means of a cam 435 mounted on the shaft 415. The pin 457 cooperating with the plate 485 limits the rotation of the shaft 455 in accordance with the band selected by the turret. A.G.C. in cascode amplifier. The grids of the valves 50, 64 are inter-connected by a resistor, the grid of valve 64 being earthed for high frequency signals through the condenser 74, and an A.G.C. signal being applied through the resistor 55 to the grid of the valve 50. Local oscillator. The local oscillator for both V.H.F. and U.H.F. working comprises effectively a valve 150 having a half-wavelength line, modified by the insertion of lumped inductances and terminated by the variable condensers, connected between its grid and anode. At V.H.F. the heater chokes 164, 165 together with the inherent capacity 310 between the cathode and heater present a low impedance and the cathode is effectively at earth potention. At U.H.F. the series resonance of the heater chokes and the capacity 310 is chosen to cause the cathode to " float," it being stated that this condition is essential for operation at U.H.F. Feedback is provided by the condenser 170 connected between the anode and heater.